How to make a bone follow location of two bones at same time?
You can achieve this, by creating a copy location constraint for the bone, which has to be between the controlers.


  The Copy Location constraint forces its owner to have the same
  location as its target.


Given are the three bones: footA, footB, torso (which has to be between footA and footB).

Make sure the torso bone isn't connected to another bone. It can have a parent though.


  Note that if you use such a constraint on a connected bone, it will
  have no effect, as it is the parent’s tip which controls the position
  of your owner bone’s root.



Add a Copy Location Constraint to torso. Set the Target to footA.
Add a Copy Location Constraint to torso. Set the Target to footB. Lower the Influence slider to 0.5 on this constraint.


The torso bone will move to footA with the first constraint. Then, with an Influence of 0.5, the torso bone will move halfway towards footB.

This can also be done for the rotation with Copy Rotation Constraints.